New Phosphoranylideneketenes and Phosphoranylidenethioketenes‐Synthesis, Structure, Spectroscopy, and Lewis Base Properties

Posphoranylideneketene Ph₃PCCO and its sulfur analogue, Ph₃PCCS, are cumulated ylides that exhibit diverse reactivity and distinct structural features. Herein, we present the first systematic study of substituent effects on the structures and bonding of a series of aryl- and alkyl-substituted phosphoranylideneketenes, R₃PCCO [R = Et, cyclohexyl, NMe₂, 4-Me₂N-C₆H₄, 4-MeO-C₆H₄, 4-CF₃-C₆H₄, 3,5-(CF₃)₂-C₆H₃], prepared from the corresponding phosphonium salts, [R₃PCH₂COOEt]X (X = Br, I) and two equivalents of Na[N(SiMe₃)₂]. Multinuclear NMR and IR spectroscopy, supported by single-crystal X-ray diffraction, revealed that the bond strength within the central P═C═C═O fragment increases with the electron-deficiency of the aryl substituents at phosphorus. The corresponding phosphoranylidenethioketenes, R₃PCCS (R = Et, cyclohexyl, NMe₂, 4-Me₂N-C₆H₄, 4-MeO-C₆H₄, 4-CF₃-C₆H₄), were obtained by reactions of R₃PCCO with CS₂. The rate of conversion of R₃PCCO into R₃PCCS decreases with increasing electron deficiency at phosphorus. Both R₃PCCO and R₃PCCS act as relatively weak ambidentate Lewis donors, yet they form stable acid-base adducts with strong Lewis acids such as B(C₆F₅)₃ and Al(C₆F₅)₃.